The Application of Thermal Decomposition for Determination of Carbonate Acid-Neutralising Capacity for Improved Acid Mine Drainage Prediction

Andrea R Gerson,George Levay,Mike Hutton-Ashkenny,Roger St C Smart,Paul Weber,Rosalind Green

doi:10.3390/min11111181

Abstract

This study developed an industry-applicable, thermal decomposition methodology for quantification of carbonate mineral acid neutralisation capacity (ANCtherm-carb) for waste rock, tailings, and other mined materials. Standard titration-based methods for ANC can be compromised due to contributions from silicate minerals, ion exchange, Fe-rich carbonates, and other transition metal carbonates. C emission (CO2 and CO) was measured using IR in a N2 atmosphere. Cneut (wt%) was calculated using the C emission at 800 or 1000 °C minus the C emission at 400, 450 or 500 °C and the weight of sample prior to decomposition (Equation (2) of this manuscript). This value was then input into Equation (3) of this manuscript to calculate ANCtherm-carb. Good correlation of ANCtherm-carb for single-mineral carbonates with ANCcalc, calculated from bulk assay concentrations for Mg, K, Na, Ca, and Mn, was achieved. Thereafter, 18 waste rock samples were examined, resulting in the correlation of ANCtherm-carb versus non-standard ANCtitrate-carb (titration methodology adapted to focus on carbonate neutralisation only) with R2 = 0.96. This correlation is valid for samples containing both non-neutralising carbonates (siderite) and sources of neutralisation arising from non-carbonates (Mg-clay) within this waste rock system. Typically, mining operations use total C measurements for assessment of carbonate neutralisation potential in the block and mining model. This method provides an effective means to cheaply analyse for carbonate neutralisation potential with assignment of potentially acid-forming and non-acid-forming blocks to waste rock cells, etc.

Highlights

Acid mine drainage (AMD) is an internationally-recognised environmental issue arising, primarily, from the exposure of pyrite (FeS2) to weathering, resulting in the formation of acid and simultaneous metalliferous drainage [1]
One of the first steps to determine if AMD is likely is the geochemical characterisation of mine waste materials
All these carbonate minerals were present in their respective samples at >90 wt% except for magnesite and ankerite

Summary

Introduction

Acid mine drainage (AMD) is an internationally-recognised environmental issue arising, primarily, from the exposure of pyrite (FeS2) to weathering, resulting in the formation of acid and simultaneous metalliferous drainage [1]. One of the first steps to determine if AMD is likely is the geochemical characterisation of mine waste materials For this characterisation process, standardised, internationally-recognised methods are applied [2]. The net acid production potential (NAPP) is defined as MPA minus ANC. If this value is positive, the waste is likely to be acid generating. This initial assessment is, thereafter, complemented by other analyses such as net acid generation testing, neutralisation potential ratio (NPR) determination and kinetic leach columns [4,5]. A NPR assessment in block/mining models is often used to assign acid-generating material, but during initial or detailed characterisation studies, it is common to explore more broadly with AMIRA International Ltd classifications, etc. [4,5]

Objectives

Methods

Conclusion